Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy into light. Improvements in these devices have resulted in their use as light sources replacing conventional incandescent and fluorescent light fixtures. The energy conversion efficiency of LEDs now approaches the level attained by fluorescent light fixtures and promises to exceed even these efficiencies. Moreover, LEDs have significantly longer lifetimes than both incandescent bulbs and fluorescent tubes. However, the useful lifetime of LEDs is significantly reduced if the operating temperature exceeds certain limits.
The operating environment of an LED light source is typically hot, and overheating must be controlled in order to extend the operating life of the light source. The high operating temperatures of commercial white LED light sources result primarily from two factors. First, the phosphor that converts blue light from the LED dies into longer wavelength light generates heat. Thin layers of Group III nitrides, such as gallium nitrides (GaN or gallium indium nitride GaInN), are used to produce LEDs for general commercial lighting applications. For example, thin epitaxial layers of gallium nitrides are grown on sapphire substrates (Al2O3). Light is emitted from the epitaxial layers sandwiched between oppositely doped layers when a voltage is applied across the doped layers. Gallium-nitride LED dies (GaN or GaInN) emit blue light having a wavelength in a range from 430 nanometers to 460 nanometers. A phosphor coating then absorbs some of the emitted blue light and fluoresces to emit light with longer wavelengths so that the overall LED device emits light with a wider range of wavelengths, which is perceived as “white” light by a human observer. The phosphor does not convert all of the blue light to longer wavelength light, but rather converts much of the blue light to heat.
Second, a single LED die produces too little light to be used as a replacement for a conventional light source in most applications. Hence, a replacement light source must include a large number of individual LED dies. The large number of LED dies that are packaged in close proximity to one another under a transparent carrier material that contains phosphor particles results in a large amount of heat generated within a small volume. The temperature under the transparent carrier material rises when the large amount of heat generated by the many LED dies cannot be conducted fast enough away from the LED device due to inadequate heat conduction of the luminaire housing, which may be exacerbated in a hot environment.
Although LED package designs include heat carriers and heat sinks that conduct heat away from the LED device, it is nevertheless advantageous to determine the temperature of the LED device in order to take corrective measures if heat is not dissipated sufficiently to maintain the temperature of the LED device below a critical level. A conventional way to determine the temperature of the LED device is to place a thermistor or thermocouple on the LED package near the LED device. However, this method does not measure the temperature directly at the LED dies covered by the transparent carrier material. Depending on how the heat propagates away from the LED dies, the temperature at the thermistor does not reflect the actual temperature under the transparent carrier material. Moreover, this manner of measuring temperature provides a relatively slow feedback and can lead to oscillation in the temperature control. Because the source of the heat is the LED dies and the phosphor particles under the transparent carrier material, the temperature at the thermistor or thermocouple outside the transparent carrier material is indicative of the heat that was produced earlier within the transparent carrier material. By the time the thermistor or thermocouple measures a temperature that exceeds a threshold and LED drive current is reduced in order to reduce the heat generated by the LED device, the temperature within the transparent carrier material may already have fallen because the temperature measured at the thermistor or thermocouple resulted from earlier produced heat that later reached the thermistor or thermocouple. The delayed feedback will cause the current control to overcompensate both after the measured temperature exceeds an upper threshold and after the measured temperature falls below a lower threshold. An oscillating LED device temperature results.
Thermistors and thermocouples are typically not placed near the LED dies under the transparent carrier material, however, because they absorb light and would result in a non-uniform pattern of light generation from the LED device. Moreover, placing a thermistor or thermocouple within the LED array would add an additional manufacturing step and would require additional machinery. So the cost of the resulting LED device would increase significantly. A inexpensive method is sought for determining the temperature of LED dies covered by a transparent carrier material that includes phosphor without causing the light emitted from the LED device to be non-uniform.